Hydrozoa. Acalephje (Cuv). Part 3

Fig. 51. Cassiopea Borbonica.

(278). The concave or ventral surface of the disk is furnished with a double investment, consisting of an outer and inner layer, the external of which resembles in its structure the dorsal membrane described above, and constitutes a sort of epidermic covering. The inner layer, which in its intimate texture likewise consists of hexagonal cells, encloses nothing but a number of isolated granules, clear and translucent as water. The interspace between this inner layer and the dorsal integument is considerably greater than that which separates it from the ventral surface; both these spaces, however, are filled up with a clear gelatinous mass, wherein are distinguishable numerous isolated granular bodies, of a rounded shape and of unequal size, that seem to be all connected with each other by fibres or extremely delicate vessels, and not supported by expansions of cellular membrane. The rest of the gelatinous mass is too transparent to allow any organization to be detected; this, however, is in small proportion, and encloses the large vessels belonging to the nutritive apparatus, immediately to be described.

(279). The opening of the mouth is situated in the centre of the lower surface of the disk, between the four arms suspended from that portion of the body. The mouth itself consists of a short quadrangular tube, from the angles of which the arms are dependent. Each arm is composed of a thick central cartilage, whereunto are attached two membranous laminae, variously plaited and puckered throughout their entire length, and moreover at certain seasons gathered into little pouches or pockets, to be hereafter mentioned in connexion with the generative apparatus.

(280). Superiorly the oral aperture terminates in four short tubes arising from its four angles; and these, diverging, mount upwards, supported by a cartilaginous prolongation derived from the central supports of the arms. These four tubes evidently represent the oesophagus and lead into four ample stomachs of a subglobular shape, which are smooth internally and lined by a special membrane, wherein may be seen numerous little granular bodies, but no vessels.

(281). From the above stomachal cavities proceed several large canals that diverge towards the circumference of the disk, and constitute a part of the digestive apparatus. One of these vessels arises immediately from the dilated portion of each oesophageal tube; and these, dividing and subdividing dichotomously, ramify towards the margin of the disk. From each of the four stomachs three other large canals take their origin, and ran in the same direction; of these, the two lateral ones are simple and unbranehed, but that in the centre ramified dichotomously. These sixteen large vascular trunks, together with all their numerous ramifications, sometimes anastomotically united, ultimately terminate in a wide circular vessel that surrounds the margin of the disk. The nutrient canals are situated beneath the inner membrane, described above, whereby they are partially enclosed and supported.

(282). Before closing our description of the alimentary system of the Pulmonigrade Acalephae, we must mention some accessory organs, of recent discovery, which are in connexion with it. Eschscholtz* describes a series of elongated granular bodies, placed in little depressions around the margin of the disk, which seem to be of a glandular nature, and apparently communicate by means of minute tubes with the nutrient canals: these he regards as the rudiments of a biliary system. Other observers assign a similar office to a cluster of blind sacculi or caeca, which are connected in some species with the commencement of the radiating tubes; it is, however, scarcely necessary to observe that such surmises relative to the function of minute parts are but little satisfactory.

(283). Prior to the publication of Ehrenberg's important researches relative to the anatomy of the Cyanea aurita1, it was generally believed that in the Pulmonigrade Medusae the alimentary canals were unprovided with any excrementitious orifices; these, however, were discovered by the illustrious Prussian observer, occupying the situations indicated by eight dark-brown-coloured spots situated at equal distances around the margin of the disk, and which had previously been suspected to be the analogues of a biliary organ. By keeping the living Medusae for some time in sea-water deeply coloured with indigo, and thus causing all the ramifications of the alimentary apparatus to become filled with the coloured fluid, while the rest of the body remained transparent and colourless, it appeared that, opposite each of the above-mentioned spots, the circular marginal canal into which the nutritive tubes, radiating from the stomach, empty themselves becomes dilated into a sort of cloacal cavity, in which the debris of digested materials, such as the shells of minute Conchifera, Botifera, Bacil-laria, etc, were easily distinguishable; from each of these cloacal dilatations, canals can readily be traced communicating with the exterior; and on irritating the living animal, it is easy to witness the discharge of excrementitious matter through the eight marginal orifices of the disk.

(284). A distinct movement is frequently perceptible in the interior of the ramifying alimentary tubes, which has been mistaken for a circulation, but which is merely the effect of ciliary action, or of peristaltic movement in the walls of the intestine.

(285). Up to the period when Ehrenberg made the important researches we are laying before the reader, relative to the anatomy of these creatures, it was impossible to account for the capability of locomotion which the Pulmonigrade Acalephae evidently possess, but which his researches serve to render perfectly intelligible. The canals formed by the ramifications of the alimentary apparatus he observed to be all bordered by two delicate lines of a pale red colour, which, under the microscope, are evidently of a muscular character; by the contractions of these, therefore, the most important movements of the animal are accomplished. Besides the above, however, other muscles are discernible. In Cyanea the disk is surrounded with a fringe of tentacula, each of which exhibits at its base a muscular structure; consequently the possession of muscular fibre is evidently established as a part of the economy of these animals.